Direct observation of carbon - Carbon bond cleavage in ultrafast decarboxylations

Full text of DOI:10.1021/ja960112j

4502
J. Am. Chem. Soc. 1996, 118, 4502-4503
We find that photoexcitation (hνCT) according to eq 1 of the
colored pyridinium salts of benzilic acid leads to the loss of
carbon dioxide from the intermediate acyloxy radical within a
picosecond. These ultrafast decarboxylations thus approach
barrier-free unimolecular rates to approximate the transition state
of C-C bond scission.11 For example, upon the photoexcitation
Direct Observation of Carbon-Carbon Bond
Cleavage in Ultrafast Decarboxylations
T. Michael Bockman, Stephan M. Hubig, and Jay K. Kochi*
Department of Chemistry, UniVersity of Houston
Houston, Texas, 77204-5641
of the yellow methylviologen salt12 of benzilic acid at λCT
)
ReceiVed January 16, 1996
375 nm, the UV absorption band of the reduced methylviologen
(A•) at λmax ) 395 nm13 is observed immediately (Figure 1). In
addition, the transient spectrum shows another visible absorption
centered at 600 nm, and the digital deconvolution of the
experimental spectrum yields the resolved absorption bands of
MV•13 and Ph2C•OH,14 as illustrated in Figure 1.15 On the basis
of the observation of both transients within 1-2 ps of the laser
excitation, we conclude that the photoinduced electron transfer
in eq 1 is closely accompanied by decarboxylation, i.e.
We wish to report how the recent developments in time-
resolved spectroscopy make it possible to observe the direct
scission of a carbon-carbon bond in real time, a process of
fundamental importance in organic chemistry.1,2 We initially
focus on the facile C-C bond cleavage in the decarboxylation
•
of labile acyloxy radicals (R-CO2 ) since they generally have
lifetimes of τ < 10-9 s.3 In order to generate this reactive
precursor, we employ the novel method based on electron-
transfer oxidation of carboxylate salts,4 i.e.
hνCT
-
•
Ph2C(OH)CO2 , MV+
8 MV•, Ph2C(OH)CO2
hνCT
kCC
-
•
R-CO2 , A+
8 A•, R-CO2
8 R• + CO2 (1)
kCC
8 Ph2C•OH + CO2 (2)
which circumvents the difficulties encountered in the usual
methodology based on bond homolysis.5 Thus the instantaneous
production of the acyloxy radical by oxidation of carboxylate
is achieved by charge-transfer excitation (hνCT)7 in eq 1 with
the aid of a new pump-probe spectrometer based on a 230-fs
high-power Ti:sapphire laser10 which allows the simultaneous
spectral detection of the reactive transients over a continuous
(350-900 nm) wavelength range.
where MV+ and MV• represent methylviologen and reduced
methylviologen,13 respectively. The absorption spectrum of
Ph2C•OH can also be generated without admixture from the MV•
spectrum by the charge-transfer photostimulation of the N-meth-
yl-4-cyanopyridinium salt, as shown in the inset of Figure 1.16
Simultaneous monitoring of the growth of both transients,
as illustrated in Figure 2, demonstrates that the formation of
Ph2C•OH is significantly slower (by about 1 ps) than the
formation of MV•. Reduced methylviologen is thus generated
with the same time constant (τ < 700 fs, fwhm) as the instru-
mental response of the laser system, in accordance with its
instantaneous generation in eq 1.7 On the other hand, the slower
rise of the ketyl radical absorption indicates that it is formed
(1) (a) Manz, J.; Wo¨ste, L. Femtosecond Chemistry; VCH: Weinheim,
1995. (b) Simon, J. D. ReV. Sci. Instrum. 1989, 60, 3597.
(2) (a) Kliner, D. A. V.; Alfano, J. C.; Barbara, P. F. J. Chem. Phys.
1993, 98, 5375. (b) Sension, R. J.; Repinec, S. T.; Szarka, A. Z.;
Hochstrasser, R. M. J. Chem. Phys. 1993, 98, 6291. (c) Joly, A. G.; Nelson,
K. A. Chem. Phys. 1991, 152, 69. (d) Schwartz, B. J.; Peteanu, L. A.; Harris,
C. B. J. Phys. Chem. 1992, 96, 3591. (e) Schoenlein, R. W.; Peteanu, L.
A.; Wang, Q.; Mathies, R. A.; Shank, C. V. J. Phys. Chem. 1993, 97, 12087.
(f) Laermer, F.; Elsaesser, T.; Kaiser, W. Chem. Phys. Lett. 1988, 148,
119. (g) Lenderink, E.; Duppen, K.; Wiersma, D. A. Chem. Phys. Lett.
1993, 211, 503. (h) Sakata, Y.; Tsue, H.; Oneil, M. P.; Wiederrecht, G. P.;
Wasielewski, M. R. J. Am. Chem. Soc. 1994, 116, 6904.
(11) Moore, J. W.; Pearson, R. G. Kinetics and Mechanism, 3rd ed.;
Wiley: New York, 1981; p 173.
(12) (a) Ebbesen, T. W.; Ferraudi, G. J. Phys. Chem. 1983, 87, 3717.
(b) Ebbesen, T. W.; Manring, L. E.; Peters, K. S. J. Am. Chem. Soc. 1984,
106, 7400. (c) Hubig, S. M. J. Phys. Chem. 1992, 96, 2903. (d) Hubig, S.
M.; Kochi, J. K. J. Phys. Chem. 1995, 99, 17578.
(13) (a) Michaelis, L. Biochem. Z. 1932, 250, 564. (b) Michaelis, L.;
Hill, E. S. J. Gen. Physiol. 1933, 16, 859. (c) Bockman, T. M.; Kochi, J.
K. J. Org. Chem. 1990, 55, 4127. (d) Watanabe, T.; Honda, K. J. Phys.
Chem. 1982, 86, 2617.
(3) (a) Braun, W.; Rajbenbach, L.; Eirich, F. R. J. Phys. Chem. 1962,
66, 1591. (b) Kaptein, R.; Brokken-Zijp, J.; de Kanter, F. J. J. J. Am. Chem.
Soc. 1972, 94, 6280. (c) Falvey, D. E.; Schuster, G. B. J. Am. Chem. Soc.
1986, 108, 7419. (d) Hilborn, J. W.; Pincock, J. A. J. Am. Chem. Soc. 1991,
113, 2683. (e) Budac, D.; Wan, P. J. Photochem. Photobiol. A 1992, 67,
135.
(4) See: (a) Barnett, J. R.; Hopkins, A. S.; Ledwith, A. J. Chem. Soc.
Perkin Trans. 2 1973, 80. (b) Deronzier, A.; Esposito, F. NouV. J. Chem.
1983, 7, 15. (c) Jones, G., II; Zisk, M. B. J. Org. Chem. 1986, 51, 947. (d)
Mandler, D.; Willner, I. J. Chem. Soc. Perkin Trans. 2 1988, 997.
(5) Bond homolysis by the direct photoexcitation of precursors such as
peroxides,3c,6 esters,3d etc. involves excited states, the temporal relaxation
of which leads to the acyloxy radical on the same time scale as the cleavage
process itself. The overall kinetics of the photodecarboxylation is thus a
convolution of the formation as well as the decomposition of the acyloxy
radical, and rate constants (kCC) on the picosecond time scale cannot be
readily extracted. For a discussion of this problem in the context of peroxide
decomposition, see ref 3c.
(6) Pacansky, J.; Brown, D. W. J. Phys. Chem. 1983, 87, 1553. See
also: Sheldon, R. A.; Kochi, J. K. J. Am. Chem. Soc. 1970, 92, 5175.
(7) Time-resolved spectroscopy by Mataga, Hochstrasser, and co-
workers8 has established charge-transfer excitation to effect the transfer of
an electron from the donor to the acceptor within 500 fs. The radical-ion
pair is thus already present in its ground state prior to the time window of
our experiments, in accord with Mulliken theory.9
(8) (a) Ojima, S.; Miyasaka, H.; Mataga, N. J. Phys. Chem. 1990, 94,
4147. (b) Wynne, K.; Galli, C.; Hochstrasser, R. M. J. Chem. Phys. 1994,
100, 4797. See also: (c) Hilinski, E. F.; Masnovi, J. M.; Amatore, C.; Kochi,
J. K.; Rentzepis, P. M. J. Am. Chem. Soc. 1983, 105, 6167. (d) Hilinski, E.
F.; Masnovi, J. M.; Kochi, J. K.; Rentzepis, P. M. J. Am. Chem. Soc. 1984,
106, 8071.
(9) (a) Mulliken, R. S. J. Am. Chem. Soc. 1952, 74, 811. (b) Mulliken,
R. S.; Person, W. B. Molecular Complexes: A Lecture and Reprint Volume;
Wiley: New York, 1969. (c) Mulliken, R. S. J. Am. Chem. Soc. 1950, 72,
600.
(14) (a) Johnston, L. J.; Lougnot, D. L.; Wintgens, V.; Scaiano, J. C. J.
Am. Chem. Soc. 1988, 110, 518. (b) Nagarajan, V.; Fessenden, R. W. Chem.
Phys. Lett. 1984, 112, 207.
(15) (a) The derived spectrum of the ketyl radical (- - -) is obtained
by subtraction of the authentic spectrum of reduced methylviologen (‚‚‚)
from the composite absorption band (s). The derived extinction coefficient
of ꢀ530 ) 6200 L mol-1 cm-1 for Ph2C•OH (based on its equimolar formation
with MV• in eq 2) is in reasonable agreement with the reported value15b of
ꢀ530 ) 5500 L mol-1 cm-1. Since the experimental spectrum in Figure 1 is
unchanged over a period of more than 50 ps, correction for group velocity
dispersion is unnecessary. (b) Hayon, E.; Ibata, T.; Lichtin, N. N.; Simic,
M. J. Phys. Chem. 1972, 76, 2072.
(16) (a) The authentic spectrum of the ketyl radical (Ph2C•OH) in the
inset of Figure 1 was obtained by picosecond charge-transfer excitation of
the ion pair formed from N-methyl-4-cyanopyridinium (NCP+) and benzilate
and is identical with the spectrum of Ph2C•OH generated by pulse-
radiolysis.15b (b) Note that the absorption spectrum (λmax ) 390 nm) of the
reduced N-methyl-4-cyanopyridinyl radical (NCP•) is strongly blue-shifted
away from the absorption band of the ketyl radical. See: Itoh, M.; Nagakura,
S. Bull. Chem. Soc. Jpn. 1966, 39, 369.
(17) (a) The transient absorbance of reduced methylviologen is simulated
simply as the integral over time of the laser profile. (b) The rise of the
530-nm absorbance is simulated by the combination of the instantaneous7
rise of the MV• absorbance and the (slower) first-order rise of the absorbance
of the ketyl radical, Ph2C•OH, with a rate constant of kCC ) 8 × 1011 s-1
)
MV• (ꢀ530 ) 3200 M-1 cm-1 13d and Ph2C•OH (ꢀ530 ) 5500 M-1 cm-1).15b
.
The ratio of the two components is based on the extinction coefficients of
(c) The kinetic trace of the ketyl radical in Figure 2 is displaced by 0.4 ps
to accommodate the difference in the group velocity dispersion of the 530-
nm monitoring wavelength relative to that at 605 nm. [Note that the later
rise of the 530-nm absorbance is opposite to that based on the consideration
of group velocity dispersion and thus cannot be due to an instrumental
artifact (“chirp”). See: Sharma, D. K.; Yip, R. W.; Williams, D. F.;
Sugamori, S. E.; Bradley, L. L. T. Chem. Phys. Lett. 1976, 41, 460. For an
example of chirp-induced spectral distortion, see: Miyasaka, H.; Ojima,
S.; Mataga, N. J. Phys. Chem. 1989, 93, 3380. Yip, R. W.; Korppi-Tommola,
J. ReV. Chem. Intermed. 1985, 6, 33.]
(10) The laser spectrometer consists of a Ti:sapphire oscillator and two
amplifiers. The oscillator is pumped by an argon ion laser to generate 110-
fs pulses tunable between 720 and 920 nm. These pulses are amplified
sequentially by a regenerative amplifier and a linear (multipass) amplifier,
both of which are pumped by the frequency-doubled output of a (10 Hz)
Q-switched Nd:YAG laser. The laser system produces 230-fs pulses with
energies up to 10 mJ which are frequency-doubled (360-460 nm) for
excitation.
S0002-7863(96)00112-6 CCC: $12.00 © 1996 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 118, No. 18, 1996 4503
quantitative analysis of the fractional residual absorbance, allows
the separation of the rate constants kCC and kbet for the systems
listed in Table 1.18
The ultrafast C-CO2 bond scission in acyloxy radicals that
are derived from benzilates, with first-order rate constants kCC
in the range of 1011-1012 s-1, stands in marked contrast to the
much slower rates measured for alkyl- and aryl-substituted
acetyloxy radicals, with kCC ≈ 109-1010 s-1 3
. (Rate constants
for the decarboxylation of these acyloxy radicals determined
by our method agree well with those reported earlier.19) These
rapid rates reflect the very low energies of the transition state
(relative to the acyloxy reactant), which in turn are a conse-
quence of the high degree of stabilization of the ketyl radical,
Ar2C•OH, as the product of decarboxylation.6
The trend in the rates of carbon-carbon bond cleavage show
a consistent decrease as more methyl and methoxy substituents
are added to the benzilate anions.20 As a result, the decarboxy-
•
Figure 1. Transient absorption spectrum obtained at 1.0 ps following
the 375-nm charge-transfer excitation of the [MV+, benzilate-] salt in
water at 25 °C. Inset: (left) Authentic spectrum of the ketyl radical
(Ph2C•OH) in ref 16 and (right) the spectrum of reduced methylviologen
(MV•) in ref 13c. Digital deconvolution of the broad absorption band
(440-740 nm) in the experimental spectrum is shown as the composite
of the spectra of Ph2C•OH (- - -) and MV• (‚‚‚), as described in
footnote 15.
lation is most rapid for the benziloxy radical (Ph2C(OH)CO2 ),
for which the electron-deficiency (hole) of the photooxidized
anion is less effectively ameliorated by the electron-poor phenyl
moiety relative to the tolyl and anisyl analogues. Furthermore,
the opposite substituent effect is expected for the stabilization
of the electron-rich product (Ar2C•OH) radicals.21 Conse-
quently, electron-donating substituents can stabilize the acyloxy
reactant as well as destabilize the ketyl product, and thus the
Me- and MeO-substituted benziloxy radicals suffer decreased
rates of decarboxylation. On the other hand, the rates for back
electron transfer (kbet) in Table 1 increase as the aromatic
systems become more electron-rich. This trend is in accord
with Marcus theory,22 which predicts such an increase in kbet
as the exergonicity of the electron transfer decreases.23
We believe that the ultrafast bond cleavages exemplified by
decarboxylation of the acyloxy radicals derived from benzilates
do not represent limiting cases with respect to the ultimate
stability of the product radical. Since increased stabilization
of R• in eq 2 leads to more rapid rates of R-CO2• scission, we
hope to design carboxylate anions that lose an electron and
cleave the C-C bond in a single step. Real-time monitoring
of these concerted reactions will thus constitute the direct obser-
vation of the transition state24 for the breaking of a C-C bond.
Figure 2. Kinetic (fs/ps) traces of the formation of MV• (b) and
Ph2C•OH (O) monitored at 605 and 530 nm, respectively. The smooth
curves represent (a) the rise of MV• simulated by taking into account
the instrumental response time of 700 fs (fwhm, see footnote 17a) and
(b) the rise of Ph2C•OH simulated by the convolution of the instrumental
response time and the decarboxylation rate constant of kCC ) 8 × 1011
s-1 (see footnote 17b). The error bars reflect an uncertainty of (0.005
absorbance units in the spectroscopic measurements.
Acknowledgment. We thank Photonics Industries for technical
assistance in the construction of the femtosecond laser system, and the
National Science Foundation, the Robert A. Welch Foundation, and
the Texas Advanced Research Program for financial support.
Table 1. Rate Constants for Decarboxylation and Back Electron
Transfer in Methylviologen/Benzilate Radical-Ion Pairsa
JA960112J
-
benzilate donor (RCO2
)
kbetb (1011 s-1
)
kCCc (1011 s-1
)
(18) (a) The quantitative analysis of the rate constants of the two
-
8d
5
1
2
0.4
competing processes for the benzilates in Table 1 except Ph2C(OH)CO2
unsubstituted
4,4′-dimethyl
4-methoxy
2
5
3
8
8
is based on the following: ΦMV ) kCC/(kCC + kbet) ) kCCτMV. The lifetime
of reduced MV (τMV) is obtained from the fs/ps kinetics, and the transient
quantum yield (ΦMV) of residual MV• is obtained by nanosecond laser
photolysis using benzophenone triplet as actinometer. See: Hurley, J. K.;
Sinai, N.; Linshitz, H. Photochem. Photobiol. 1983, 38, 9. The escape of
the radicals out of the solvent cage is not included in the femtosecond
kinetics, since it occurs on much slower time scales. See: Scott, T. W.;
Liu, S. N. J. Phys. Chem. 1989, 93, 1393. (b) For Ph2C(OH)CO2-, the
value of kCC was directly obtained from the rise of the ketyl radical (see
Figure 2), since back electron transfer is negligible as described in ref 17.
(19) Earlier observations were mostly obtained by indirect methods3d
calibrated by a single direct measurement3c on the picosecond time scale.
(20) The effects of electron-withdrawing substituents such as chloro could
not be examined, since the charge-transfer absorption band of the [MV+,
4,4-dichlorobenzilate] ion pair was blue-shifted to λCT < 360 nm and thus
could not be excited with the Ti:sapphire laser system (see footnote 10).
(21) Ketyl radicals are electron-rich by virtue of their very low oxidation
potentials21a and ease of oxidation in solution. See: (a) Baumann, H.;
Merckel, C.; Timpe, H.-J.; Graness, A.; Kleinschmidt, J.; Gould, I. R.; Turro,
N. J. Chem. Phys. Lett. 1984, 103, 497. (b) Kemp, T. J.; Martins, L. J. A.
J. Chem. Soc. Perkin Trans. 2 1980, 1708. (c) Engel, P. S.; Wu, W. X. J.
Am. Chem. Soc. 1989, 111, 1830.
4,4′-dimethoxy
2,2′,5,5′-tetramethoxy
a Benziloxy radicals generated by charge-transfer irradiation of
[MV+, RCO2-] with the 230-fs (fwhm) laser pulse at λexc ) 375 or
400 nm in aqueous solution at 23 °C. b Rate constant ((5%) for back
electron transfer in eq 3. c Rate constant ((5% unless indicated
otherwise) for decarboxylation of the acyloxy radical derived from the
benzilate anion in eq 3. d Rate constant ((10%) for benziloxy only.
from the short-lived benziloxy precursor. Simulation and
comparison of the kinetic traces in Figure 2 allow the extraction
of the first-order rate constant for the formation of the ketyl
radical, and thus kCC in eq 2 is 8 × 1011 s-1 17
.
Polar (substituent) effects on the C-C bond cleavage are
examined in the series of methylviologen salts of methyl- and
methoxy-substituted benzilates. The kinetic traces show partial
decay of the spectral transients to a nonzero residual absorbance,
the value of which varies with the particular benzilate anion.
The complex transient kinetics can be analyzed in terms of a
competition between C-C bond cleavage (eq 2) and back
electron transfer (kbet) to restore the original viologen salt, i.e.
(22) (a) Marcus, R. A. Annu. ReV. Phys. Chem. 1964, 15, 155. (b) Marcus,
R. A. J. Chem. Phys. 1965, 43, 679. (c) Marcus, R. A. Faraday Discuss.
Chem. Soc. 1982, 74, 7. (d) Marcus, R. A. J. Chem. Phys. 1956, 24, 966.
(23) (a) This trend in kbet is termed “inverted” to contrast with the
“normal” behavior of increasing reaction rate with increasing thermodynamic
driving force. Inverted driving-force dependence of kbet has been observed
with methylviologen complexes12c,d and with a variety of other electron
donor-acceptor complexes.23b (b) Asahi, T.; Ohkohchi, M.; Mataga, N. J.
Phys. Chem. 1993, 97, 13132 and references therein.
kbet
kCC
-
•
R-CO2 , A+ 7 A•, R-CO2
8 R• + CO2 (3)
Simulation of the rise and decay of MV•, combined with
(24) Polanyi, J. C.; Zewail, A. H. Acc. Chem. Res. 1995, 28, 119.